Reagent dosing system and method of dosing reagent

ABSTRACT

A method of dosing a reagent into an exhaust gas stream of an internal combustion engine having an SCR catalyst, the method comprising injecting reagent from a reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a first dosing schedule in order to remediate a predetermined proportion of NOx in the exhaust gas stream, the first dosing schedule being associated with a first range of engine operating conditions; and injecting reagent from the reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a second dosing schedule in order to enable heat transfer between the reagent injector and said injected reagent, the second dosing schedule being associated with a second range of engine operating conditions. 
     Dosing in accordance with said first or said second dosing schedule is carried out in dependence on whether engine operating conditions lie within said first or said second range of engine operating conditions, and the proportion of NOx in the exhaust gas stream which is remediated by dosing using said second dosing schedule is less than said predetermined portion. A reagent dosing system is also provided for dosing a reagent into the exhaust gas stream of an internal combustion engine, comprising a reagent tank for storing a supply of reagent, an injector module comprising an atomising dispenser and a positive-displacement metering pump which draws reagent from the reagent tank and delivers it to the dispenser a supply line coupling the reagent tank to the injector module, and a dosing control unit operable to control the injector module to inject reagent into the exhaust gas stream. A priming pump is provided to urge reagent along the supply line toward the injector module under selected conditions.

TECHNICAL FIELD

The present invention relates to a reagent dosing system for dosing areagent into the exhaust gas stream of an internal combustion engine andto a method of dosing a reagent into the exhaust gas stream of aninternal combustion engine.

BACKGROUND TO THE INVENTION

It is well known that internal combustion engines can produce harmfulchemical species in their exhaust streams. It is therefore desirable toeliminate or at least reduce such pollutants to levels low enough thathuman health is not adversely affected. As a result of the hightemperatures that are reached during a combustion event, many chemicalspecies are produced from the oxidation of hydrocarbon fuels, includingthe oxides of nitrogen (NO and NO₂, collectively referred to as NO_(x)).Due to their impact on human health, many countries in the globalcommunity have enacted legislation that seeks to limit the emission ofNOx from both mobile and stationary sources, and many techniques havebeen developed to achieve this objective. Among these, the use ofcatalysis technology has been found to be particularly effective andeconomically viable, however, it should be noted that differentapproaches are needed when treating the oxygen-rich exhaust streams fromso-called “lean” combustion than is the case for stoichiometriccombustion exhaust streams. Examples of lean combustion NOx sourcesinclude the compression ignition or diesel engine and thedirect-injected lean-burn spark ignition or gasoline engine.

Lean-burn engines are unable to take advantage of the well developed andeffective 3-way catalyst systems that are universally used byhomogeneous spark ignition engines. Accordingly, the remediation of NOxfor lean-burn engines requires the addition of a reductant inconjunction with a suitable catalyst. The reduction of NOx requires nearreal-time dosing control since NOx production closely follows engineload but is moderated by the amount of ammonia already stored on thecatalyst. Accordingly, the reductant dosing schedule is a highly dynamicactivity.

Under steady state operating conditions, with a warmed-upengine/catalyst system, it is relatively easy to match the reductantdosing rate to the engine NOx production rate and thereby achieve veryhigh conversion ratios of about 98%. However, under transient operatingconditions, achieving this match is much more challenging due tocatalyst temperature variation and NH₃ storage effects such thatconversion ratios of 85 to 90% are more typically achieved. Therefore,quantitative accuracy in dosing and responsiveness to load changes arekey requirements for such a system.

One very effective technology for the remediation of NOx in anoxygen-rich exhaust stream is the technique widely known as SelectiveCatalytic Reduction (hereafter referred to as SCR). In this approach, anammonia-containing reagent (or reductant) is injected into an exhauststream at a rate closely related to the instantaneous NOx content ofthat stream wherein the ammonia (NH₃) reacts with the NOx in conjunctionwith a vanadia-based or similar catalyst such that the pollutant isconverted to harmless nitrogen (N₂) and water in the tail gas. Bothselective catalytic reduction and selective non-catalytic reduction(SNCR) have been used extensively in the industrial sector, and SCRsystems have recently been subject to development for mobile emissionsources.

Notwithstanding the above, existing SCR dosing systems have a variety ofshortcomings.

In prior art systems, a pressurizing pump is located in or near the tankmodule in order to supply reductant at a fixed known pressure to aremote injector nozzle adjacent the SCR catalyst. This system operatesaccording to a so-called pressure/time metering principle wherebyreagent metering is achieved by exposing the control orifice to thecontrolled pressure for a known time duration. In order to achieve thenecessary stability of pressure control in such a system requiressophistication, and therefore expense. Moreover, metering accuracy isdependant on stability of the atomizing nozzle flow area which, due toits location in the hot exhaust environment, is susceptible to changedue to crystallized urea deposit build-up.

Many known systems utilise return flow architectures whereby surplusreagent above and beyond that which is needed for SCR dosing is suppliedto the nozzle purely for cooling purposes, whence it is returned througha separate duct back to the storage tank. Additionally, such systems mayrequire a purge feature which is activated on engine shut-down whichminimizes the propensity for nozzle clogging from salt precipitates dueto heat soak-back into the nozzle assembly from the hot exhaust. In suchcases, a separate purge pump is required and, since the purge is carriedout after the engine is switched off, the purge pump causes anundesirable drain on the vehicle battery.

Examples of existing dosing systems having these features are describedin WO 2006/050547 (Pankl Emission Control Systems) and WO 2008/006840(Inergy Automotive Systems Research).

Positive displacement pumps for reagent dosing are known in the priorart, for example in WO 2005/024232 (Hydraulic Ring), WO 2007/071263(Grundfos) and WO 2008/031421 (Thomas-Magnete). However, such pumps havebeen designed to operate at undesirably low pressures and, because thepumping plunger is separated from the reagent by a flexible diaphragm,the accuracy of reagent volumetric quantity metering is also of a muchlower order than can be achieved by a piston pump.

It is an object of the present invention to provide an SCR dosing systemwhich substantially overcomes or mitigates the aforementioned problems.It is a further object of the invention to provide an advantageousmethod of operating an SCR dosing system.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is provideda method of dosing a reagent into the exhaust gas stream of an internalcombustion engine having an SCR catalyst, the method comprising:

injecting reagent from a reagent tank into the exhaust gas stream at aposition upstream of the SCR catalyst using a reagent injector inaccordance with a first dosing schedule in order to remediate apredetermined proportion of NOx in the exhaust gas stream, the firstdosing schedule being associated with a first range of engine operatingconditions;

injecting reagent from the reagent tank into the exhaust gas stream at aposition upstream of the SCR catalyst using a reagent injector inaccordance with a second dosing schedule in order to enable heattransfer between the reagent injector and said injected reagent, thesecond dosing schedule being associated with a second range of engineoperating conditions; and

dosing in accordance with said first or said second dosing schedule independence on whether engine operating conditions lie within said firstor said second range of engine operating conditions;

wherein the proportion of NOx in the exhaust gas stream which isremediated by dosing using said second dosing schedule is less than saidpredetermined portion.

Typically, the second range of engine operating conditions ischaracteristic of higher engine load and/or speed than said first rangeof engine operating conditions.

The injector typically comprises an atomising dispenser and apositive-displacement metering pump, the method further comprisingdrawing reagent from the reagent tank by means of the metering pump anddelivering it to the dispenser.

The injector may comprise thermal insulation means to ensure that duringthe first dosing schedule heat transfer between the injector and theinjected reagent is sufficient for there to be no requirement foradditional dosing for the purposes of cooling.

In one embodiment, the method further comprises sensing the level ofreagent in the reagent tank, and halting the first dosing schedule whenthe sensed level of reagent drops below a predetermined level.

According to a second aspect of the present invention, there is provideda reagent dosing system for dosing a reagent into the exhaust gas streamof an internal combustion engine, the system comprising:

a reagent tank for storing a supply of reagent;

an injector module comprising an atomising dispenser and apositive-displacement metering pump which draws reagent from the reagenttank and delivers it to the dispenser;

a supply line coupling the reagent tank to the injector module;

a dosing control unit operable to control the injector module to injectreagent into the exhaust gas stream; and

an additional priming pump arranged, in use, to urge reagent along thesupply line toward the injector module.

In either of the aforementioned aspects of the invention the dispensermay be closely-coupled to the metering pump and, typically, thedispenser is integrated in the same unit as the metering pump.

The priming pump may be arranged, in use, to urge reagent along thesupply line toward the injector module at or during a start-up modeand/or to urge reagent along the supply line toward the injector modulecontinuously or intermittently during running of the engine.

A pressure sensor may be located within the supply line, and in oneembodiment the priming pump may be operable in response to a pressuresignal from the pressure sensor.

The system may further comprise a tank module mountable within thereagent tank, said tank module comprising:

a reservoir comprising a tubular member with a first open end and asecond closed end, the second end having an opening therein to enablethe inflow of reagent from the reagent tank;

a closure member for closing the first end of the tubular member toprevent the flow of reagent therefrom; and

a reagent pickup tube having a first end disposed within said reservoirand a second end in fluid communication with said supply line;

wherein the priming pump is disposed within the tank module.

Conveniently, said dosing control unit is operable to transmit andreceive data to/from an engine control unit, the engine control unitbeing operable to control an internal combustion engine to which thedosing system is installed. The dosing control unit may be operable totransmit and receive data to/from said engine control unit via a CANlink.

Alternatively, said dosing control unit may be physically integratedwith an engine control unit, the engine control unit being operable tocontrol an internal combustion engine to which the dosing system isinstalled.

In one example, there is provided an exhaust system for an internalcombustion engine comprising first and second groups of combustionchambers, the system comprising:

a first exhaust branch for receiving an exhaust gas stream emitted fromsaid first group of combustion chambers; and

a second exhaust branch for receiving an exhaust gas stream emitted fromsaid second group of combustion chambers;

wherein each one of said first and second branches comprises;

a hydrocarbon doser for injecting hydrocarbons into the exhaust gasstream;

an oxidation catalyst disposed downstream of the hydrocarbon doser;

a particulate filter disposed downstream of the oxidation catalyst;

at least one SCR doser disposed downstream of the particulate filter forinjecting reagent into the exhaust gas stream; and

an SCR catalyst disposed downstream of the SCR doser.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches, the common exhaust portioncomprising a particulate filter and said first and second branchescomprising first and second SCR catalysts, respectively; and

an SCR doser for injecting reagent into the exhaust gas stream at alocation disposed between said particulate filter and said second end ofthe common exhaust portion.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches, the common exhaust portioncomprising a particulate filter and said first and second branchescomprising first and second SCR catalysts, respectively;

a first SCR doser for injecting reagent into said first branch at alocation disposed upstream of said first SCR catalyst; and

a second SCR doser for injecting reagent into said second branch at alocation disposed upstream of said second SCR catalyst.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches;

said first branch comprising a first particulate filter, a first SCRcatalyst and a first SCR doser for injecting reagent into said firstbranch at a location disposed between said first particulate filter andsaid first SCR catalyst; and

said second branch comprising a second particulate filter, a second SCRcatalyst and a second SCR doser for injecting reagent into said secondbranch at a location disposed between said second particulate filter andsaid second SCR catalyst.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion comprising a particulate filter and having afirst end coupled to the engine for receiving an exhaust gas streamemitted therefrom and a second end which divides into first and secondbranches;

wherein the first branch comprises an SCR catalyst and an SCR doser forinjecting reagent into said first branch at a location disposed upstreamof said SCR catalyst; and

the second branch comprises a Lean NOx Trap and a hydrocarbon doser forinjecting hydrocarbons into said second branch at a location disposedupstream of said Lean NOx Trap.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches;

wherein the first branch comprises a first particulate filter, an SCRcatalyst and an SCR doser for injecting reagent into said first branchat a location disposed between said first particulate filter and saidSCR catalyst; and

the second branch comprises a second particulate filter, a Lean NOx Trapand a hydrocarbon doser for injecting hydrocarbons into said secondbranch at a location disposed between said second particulate filter andsaid Lean NOx Trap.

The exhaust system may comprise a 3-way valve disposed at said secondend of said common exhaust portion, said valve being operable to directsaid exhaust gas stream along said first and/or said second branch.

In another example, an exhaust system for an internal combustion enginecomprises

a common exhaust portion having one end coupled to the engine forreceiving an exhaust gas stream emitted therefrom;

a hydrocarbon doser for injecting hydrocarbons into said common exhaustportion;

a Lean NOx Trap disposed downstream of said hydrocarbon doser;

an SCR doser disposed downstream of said Lean NOx Trap for injectingreagent into said common exhaust portion; and

an SCR catalyst disposed downstream of said SCR doser.

According to another aspect of the present invention, there is providedan exhaust system for an internal combustion engine comprising:

a common exhaust portion having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches;

a 3-way valve disposed at said second end of said common exhaustportion, said valve being operable to direct said exhaust gas streamalong said first and/or said second branch;

wherein the first branch comprises a hydrocarbon doser for injectinghydrocarbons into said first branch and a Lean NOx Trap disposeddownstream of said HC doser;

the second branch comprises an SCR doser for injecting reagent into saidsecond branch and an SCR catalyst disposed downstream of said SCR doser;and

a downstream end of said first branch is coupled to said second branchat a location disposed upstream of said SCR doser, to enable the flow ofexhaust gases therebetween.

According to another aspect of the present invention, there is provideda method of dosing a reagent into the exhaust gas stream of an internalcombustion engine having an SCR catalyst, the method comprising:

injecting reagent with a spray momentum into the exhaust gas streamusing an injector module comprising a positive-displacement meteringpump and an atomising dispenser;

controlling the positive-displacement metering pump in accordance with acontrol parameter which is dependent on the exhaust gas stream spacevelocity so that the spray momentum provides optimum mixing of theinjected reagent with the exhaust gas stream upstream of the SCRcatalyst.

Typically, but not necessarily, said control parameter is a drivecurrent level.

It will be appreciated that preferred and/or optional features of anyaspect of the invention may be incorporated alone or in appropriatecombination in any other aspect of the invention also.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which;

FIG. 1 shows an SCR dosing system which may be used to implement methodsof the present invention;

FIG. 2 is an alternative representation of the SCR dosing system of FIG.1;

FIG. 3 is a sectional view of an injector module of the system of FIGS.1 and 2;

FIG. 4 shows an embodiment of an SCR dosing system according to thepresent invention comprising a priming pump;

FIG. 5 shows a variation of the SCR dosing system of FIG. 4 comprising astand-alone dosing control unit;

FIG. 6 shows a variation of the SCR dosing system of FIG. 4 comprisingan integrated engine/dosing control unit;

FIG. 7 shows a variation of the SCR dosing system of FIG. 6 comprisinghydrocarbon dosing control;

FIG. 8 shows a first exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 9 shows a second exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 10 shows a third exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 11 shows a fourth exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 12 shows a fifth exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 13 shows a 3-way valve for use in the exhaust system shown in FIG.12;

FIG. 14 shows a sixth exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 15 shows an example of an exhaust system comprising an SCR dosingsystem, by way of background to FIG. 16;

FIG. 16 shows a seventh exhaust system comprising an SCR dosing systemaccording to the present invention;

FIG. 17 is a graph of engine load versus engine speed and shows athermal management zone; and

FIG. 18 shows logic pulse, drive current and plunger motion for aninjector module of an SCR dosing system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, the SCR dosing system 1 is operable toinject a quantity of reagent (or reductant) into the exhaust gas streamemitted from an internal combustion engine 3. The internal combustionengine 3 is a diesel fuelled compression-ignition combustion engine.

The internal combustion engine 3 is controlled by an engine control unit(ECU)₅. An exhaust system 7 is coupled to the internal combustion engine3 for conveying exhaust gas emissions therefrom.

The exhaust system 7 comprises first, second, third and fourth portions8, 10, 12, 14. The exhaust system 7 also includes a diesel oxidationcatalyst 9, disposed between the first and second exhaust portions 8,10, a diesel particulate filter 11 (DPF), disposed between the secondand third exhaust portions 10, 12, and a selective catalytic reduction(SCR) catalyst 13, disposed between the third and fourth exhaustportions 12, 14.

During operation of the internal combustion engine 3, exhaust gasemissions from the engine enter the first exhaust portion 8 and passthrough the oxidation catalyst 9, the DPF 11 and the SCR catalyst 13,before exiting via the fourth exhaust portion 14.

The SCR dosing system 1 generally comprises a tank module 20, aninjector module 40 and a dosing control unit (DCU) 70, each of whichwill now be described in more detail.

The tank module 20 is installed within a reagent tank 21, which stores asupply of a suitable reagent (or reductant), such as aqueous urea.Aqueous urea is also known as “ADBLUE”® and as “Diesel Emissions Fluid”and, in the present embodiment, the reagent is a eutectic solution ofaqueous urea (32.5% by weight of urea in water). The reagent tank 21 maybe formed from a suitable plastic polymer, such as high densitypolyethylene (HDPE).

The tank module 20 comprises a deep drawn stainless steel cup orreservoir 30 and a polymer closure or cap 32. The cup or reservoir 30may alternatively be made from a suitable polymer or plastics material.

The tank module 20 also includes all or some of a reagent heater 22, areagent filter 23, a reagent level sensor 24, a reagent temperaturesensor 25, a reagent quality sensor 26 and a reagent pickup tube 27, allof which may be integral with the cap 32. The reagent temperature sensor25 may be, for example, a thermistor. The electrical connections for thereagent heater 22, as well as those for the reagent level, temperatureand quality sensors 24, 25, 26 pass through the cap 32.

The base of the reservoir 30 is pierced to form a hole which allowsreagent in the reagent tank 21 to flow into the tank module 20. The holemay be fitted with a one-way valve which permits reagent to flow intothe tank module 20, but prevents it from flowing back into the reagenttank 21.

The function of the reservoir 30 is to provide a structurally stablehousing for the reagent filter 23, reagent heater 22 and reagent levelsensor 24, and to provide a container in which the melted reagent may beheld following a cold soak start-up. This is necessary to meet therequirement that the dosing system 1 should be fully functional within aspecified time interval from engine start-up, even under conditionswhere the reagent may be frozen (below −11° C. in the case of aqueousurea).

The tank module 20 may be mounted in any convenient location thatpermits accessibility for periodic servicing. The reagent tank 21 may beintegrated into a vehicle fuel (hydrocarbon) tank, such that it issubmerged in the fuel or otherwise co-located, or it may be astand-alone module. The reagent tank 21 may be pressurised, for example,by engine intake boost air pressure.

As the reagent tank 21 becomes depleted, but ideally before it becomestotally exhausted, the contents must be replenished. This may beachieved by means of any suitable replenishment strategy. For example,the reagent tank 21 may have its own discrete fill port or it may beassociated with a combined vehicle fuel/reagent co-fuelling system suchas that disclosed in U.S. Pat. No. 6,554,031 (Ford Global Technologies).Other replenishment strategies include cartridge refills at vehicleservice appointment intervals and informal fill-ups which the tankfiller must accommodate.

A first end of the reagent pickup tube 27 is disposed adjacent to thereagent filter 23, such that reagent in the reservoir 30 passes throughthe reagent filter 23 as it enters the pickup tube 27. A second end ofthe pickup tube 27 terminates at a wall of the reagent tank 21.

The injector module 40 is coupled to the reagent tank 21 by means of asupply line 35. A first end of the supply line 35 is attached to thesecond end of the reagent pickup tube 27 by any suitable connectingmeans. A second end of the supply line 35 is connected to the injectormodule 40 for delivering reagent thereto.

The supply line 35 is provided with a supply line heater 37 forelevating the temperature of reagent within the supply line 35 when theambient temperature falls below minus 11 degrees Celsius. The supplyline heater 37 may be an electrically resistive element embedded withinthe supply line 35. Alternatively the element may be coiled around theexternal surface of the supply line 35. It will be appreciated by thoseskilled in the art that any suitable heating means could be employed inorder to heat the supply line 35.

Although in FIG. 2, the reagent pickup tube 27 is shown coming out ofthe bottom of the reservoir 30, the reagent pickup tube 27 may,alternatively, pass through the cap 32, with one end extending into thereservoir 30 and the other end connected to the supply line 35.Alternatively, the reagent pickup tube may pass through a side wall ofthe reservoir 30.

The injector module generally comprises an electrically actuatedvariable frequency fixed stroke positive displacement piston pump thatdelivers discrete fixed-volume parcels of reagent to a close-coupledatomizing nozzle.

Referring to FIG. 3, the injector module 40 comprises a metering pump 41and a dispenser (or atomiser) 42, which are integrated within the sameunit and coupled together by a connecting pipe 43. The metering pump 41and atomiser 42 are closely coupled and form an integrated unit. Theinjector module 40 is mounted to the third exhaust portion 12 of theexhaust system 7, upstream of the SCR catalyst 13. The dispenser 42 isdisposed within the flow of exhaust gases in the exhaust system 7 and isarranged at such an attitude that its spray cooperates with the exhaustflow to give optimum mixing between gas and reagent. The metering pump41 is disposed outside the exhaust system 7 so that it may benefit fromexposure to ambient cooling air.

The connecting pipe 43 comprises a tube 44 having a bore 45 throughwhich reagent can pass. The tube 44 is capable of accommodating reagentat high pressure. The tube 44 is received within a jacket 46 for theconnecting pipe 43 which defines a compartment 47 between the tube 44and the jacket 46. The compartment 47 is evacuated to limit the transferof heat from the hot exhaust gases within the third exhaust portion 12to the reagent in the bore 45 of the tube 44 so as to preventoverheating of the reagent.

The metering pump 41 comprises an inlet passage 48 and a filter chamber49, disposed in fluid communication with, and downstream of, the inletpassage 48. The filter chamber 49 accommodates a reagent filter 50.

The metering pump 41 also includes an actuator arrangement comprising apole element 51, a coil former 52 and a solenoid coil 53. The poleelement 51 comprises a generally cylindrical inner pole piece 54, anoutwardly-directed flange 55 and a central tubular land or projection 56situated downstream of the flange 55. The pole element 51 includes anaxial bore 57. A plunger 58 is slidably accommodated within the bore 57.

The coil former 52 is disposed around the inner pole piece 54 of thepole element 51, and a supply passage 59 is defined between the coilformer 52 and the inner pole piece 54.

The coil 53 is in electrical communication with a power supply (notshown) by way of a supply cable 60. The power supply is capable ofsupplying a variable current to the coil 53 so as to induce a variablemagnetic field around the coil 53.

A disc-shaped armature 61 is attached to an upstream end of the plunger58. A delivery valve 62 is provided downstream of the plunger 58. Apumping chamber 63 is defined between the downstream end of the plunger58 and the delivery valve 62.

In use, reagent flows from the inlet passage 48 through the reagentfilter 50, which serves to filter solid particles such as precipitatesout of the reagent flow. Thereafter the reagent can flow past thearmature 61 into the pumping chamber 63 via the supply passage 59. Inorder to dispense reagent, a current is passed through the coil 53 toenergise the coil 104 and induce a magnetic field around the coil 53.The resulting magnetic field exerts a force on the armature 61 which, inturn, drives a pumping stroke of the plunger 58. The volume of thepumping chamber 63 is reduced due to the movement of the plunger 58, sothat the pressure of the reagent in the pumping chamber 63 increases.When the pressure in the pumping chamber 63 reaches a threshold value,reagent is expelled through the delivery valve 62, thereby increasingthe pressure of reagent in the tube 44 of the connecting pipe 43.

The dispenser 42 comprises a nozzle valve 64, in the form of anoutwardly opening ‘poppet’ valve. The nozzle valve 64 comprises a valveelement 65 which is biased by means of a spring toward a non-injectingposition. When the pressure of the reagent in the tube exceeds athreshold value, the valve element is forced into an injecting positionand reagent is expelled from the dispenser.

When the plunger 58 of the pump 41 reaches the end of its pumpingstroke, pressure changes take place within the injector module 40 sothat the nozzle valve 64 and the delivery valve 62 close and theexpulsion of reagent through the dispenser 42 stops. When the currentflow through the coil 53 is switched off, the magnetic field around thecoil 53 diminishes. The magnetic force acting on the plunger 58, by wayof the armature 61, diminishes and a return spring 66 biases the plunger58 in the upstream direction. As the volume of the pumping chamber 63increases, reagent can flow into the pumping chamber 63 ready for thenext pumping stroke.

The injector module 40 is described in more detail in the Applicant'sco-pending European Patent Publication No. 1878920, the contents ofwhich are incorporated herein by reference.

The above-described metering pump 41 employs a positive displacementmetering principle as opposed to a pressure/time metering method. Morespecifically, by driving a plunger 58 of known diameter through a knownand fixed stroke, a discrete known and invariable (fixed) volume ofreagent is displaced. This results in the metering of a highlyrepeatable and consistent volumetric quantity of reagent.

Since the objective of the SCR dosing system 1 is to eliminate NOx fromthe exhaust gases downstream of the SCR catalyst 13, reagent dosingrequires a high order of volumetric accuracy since too low a dosing ratewill result in NOx breakthrough and too high a rate will result inammonia slip past the catalyst. Both of these outcomes are highlyundesirable. The positive displacement metering principle employed inthe present invention meets the requirement for accuracy of dosing in asimple and effective manner. Also for accuracy in dosing, it ispreferable to have a hydraulically “stiff” system between the meteringpump 41 and the atomizing dispenser 42 and this is achieved in theabove-described embodiment by having these two units relatively closelycoupled within an integrated unit so that the internal dead volume, andthus system compliance, is low.

An advantage of the above-described injector module relative to theprior art is that, by virtue of the positive displacement meteringprinciple, a separate pressurizing pump located in or near the tankmodule is not required. This is because the action of the metering pump41 draws reagent along the supply line from the reagent tank 21.Furthermore, the high pressure generated by the metering pump 41 at thenozzle valve 64 of the dispenser 42, e.g. of the order of 50 bar, issufficient to keep the nozzle valve 64 free from crystallized urea,thereby obviating the need for a purge system after engine shut-down.Also, by keeping the nozzle valve 64 clear, the quantity of reagentdelivered with each pumping stroke of the plunger 58 can be keptconstant.

Another advantage of the above-described injector module 40 is that themetering pump 41 does not require an intermediate diaphragm between thepumping plunger 58 and the reagent. Accordingly, the positivedisplacement metering pump 41 in the present invention can generatehigher pressures, and deliver a desired volumetric quantity of reagentwith greater accuracy, than conventional positive displacement pumps.

Variation in reagent flow rate to match the engine NOx (or otherpollutant) production rate can be made by varying the pump strokerepetition rate (pumping frequency) in unit time. By way of example, ifthe metering pump displacement per stroke is 4 mm³, then at anoperational frequency of 5 Hz the pump will displace 1.2 ml/minute ofreagent (4×5×60/1000), and this will scale linearly with frequency sothat at 100 Hz the displaced volume will be 24 ml/minute. This highlylinear response of delivery to pumping frequency is also advantageousfor simplification of the control logic.

A further advantage of the above-described injector module 40 is thetransient response or change of rate of dosing flow which can beobtained. More specifically, the metering pump 41 may be driven at arepetition rate or frequency of 0 Hz (zero flow) and then immediately goto maximum repetition rate of, for example, 150 Hz and then back to 0Hz. Although some pressure/time metered dosing systems may alsodemonstrate this functionality, particularly if the dosing (metering)valve is close to the nozzle, the frequency controlled metering pump 41in the present invention can additionally be driven in a “burst” mode inwhich it may be over-driven to, for example, 175 Hz in order to addressa short term transient requirement.

Yet another advantage of the hydraulically stiff positive displacementmetering concept is the ability to develop high injection pressure andimpart high momentum to the atomized reagent. The high injectionpressure is beneficial in achieving the fine reagent atomization that isdesirable for mixing in the exhaust flow, and for dislodgingcrystallized reagent from the nozzle that might otherwise clog orderange the spray pattern. Thus, with this configuration, the need for apurge circuit is avoided.

Still another advantage of the positive displacement concept is thataccurate totalization of reagent flow over time becomes possible if arunning tally of valid executed pump strokes is kept in memory. Thisinformation can be used to either negate the need for the reagent levelsensor 24 in the reagent tank 21, or to supplement the information froma low cost low resolution level sensor, or to provide supportinginformation to an OBD (On-Board Diagnostics) module.

As explained previously, the purpose of the dispenser 42 is to atomizethe reagent as finely and uniformly as possible and to project it with aspray pattern that enables it to mix with and evaporate in the exhaustgas stream in such a manner that, irrespective of the exhaust flow rate,the conversion from reagent to ammonia occurs in the distance betweennozzle 64 and SCR catalyst 13 and that the ammonia be presented to theSCR catalyst 13 uniformly across its face. This requires the spraypattern to match and cooperate with the particular exhaust systemgeometry immediately upstream of the SCR catalyst 13.

Achievement of these objectives is highly desirable, since it permitsboth the nozzle-to-catalyst distance to be shorter and the catalyst sizeto be smaller than would be the case for an imperfect atomizing nozzle.In some cases, the atomizing nozzle will be used in conjunction with a“mixer” device in the exhaust pipe which enhances reagent mixing withthe exhaust stream by converting the laminar gas flow into turbulentflow prior to the catalyst, and in other cases a “hydrolysis” catalystmay be employed immediately ahead of the SCR catalyst 13 to aid in thereduction of urea to ammonia.

The metering pump-injector module in the present invention may be usedwith or without the mixing aids described above.

A further advantage of the hydraulically stiff positive displacementmetering system is its potential for delivering very small quantities ofreagent directly into the combustion zone of the engine so that thebenefits of SNCR may be exploited. This is described in more detail inthe Applicant's U.S. Pat. No. 6,679,200. In SNCR, NOx reduction takesplace without the aid of a catalyst but only over a relatively narrowtemperature window that occurs immediately following the combustionevent. This however takes place under all conditions, even when theengine is cold and the SCR system is not yet functional. Therefore thepotential exists to advantageously combine the two NOx reductiontechniques to achieve an overall conversion ratio higher than either onemethod alone. Thus, both SCR and SNCR techniques may be employed usingan injector module 40 of the kind described above and, advantageously,multiple injector modules may be controlled by the same DCU.

Yet another advantage of the above-described system is that it is of asingle fluid architecture in which only aqueous urea (or other type ofreagent) is dispensed. This eliminates the need for a second fluid, suchas pressurized air, which is used in prior art systems to assist theatomization of the aqueous urea into the NOx-laden exhaust stream, andalso to help resist the formation of crystallized salts which can clogand otherwise derange the system operation.

The DCU 70 controls the reagent dosing strategy and may compriseprocessing means, such as a microprocessor, and memory means, forstoring the required software control strategies. The DCU 70 receivessignals from various engine and vehicle-mounted sensors and uses theseinputs to compute an appropriate output to the metering pump 41.Additionally, the DCU 70 controls the tank module reagent heater 22 andthe supply line heater 37.

The DCU 70 receives inputs from the reagent level sensor 24, the reagenttemperature sensor 25 and the reagent quality sensor 26. Furthermore,the SCR dosing system 1 includes a NOx sensor 72, an SCR catalysttemperature sensor 74 and an ammonia (NH₃) sensor 76. The NOx sensor 72is disposed in the second exhaust portion 10, between the oxidationcatalyst 9 and the DPF 11. The SCR catalyst temperature sensor 74 andthe NH₃ sensor 76 are disposed adjacent to the SCR catalyst 13.

In the present embodiment, the DCU 70 and the ECU 5 are linked via a CANlink 77, or other suitable connection, which enables data to betransferred therebetween. The DCU 70 also has an antenna 78 for sendingand receiving data to/from external sources.

Depending on the particular system architecture and hardwarefunctionality, the DCU 70 may be configured to perform one or more ofthe following;

a) NOx estimation model for engine exhaust. For example, the DCU 70 mayreceive data relating to engine operating conditions from the ECU 5 viathe CAN link 77 and compare the received data to pre-stored data inorder to estimate engine NOx mass production.b) NOx sensor monitoring from upstream and/or downstream (of catalyst).For example, the NOx sensor 72 may output a signal indicative of the NOxconcentration in the exhaust gas stream. An additional NOx sensor may bedisposed in the fourth exhaust portion 14 in order to detect NOx slipdownstream of the SCR catalyst 13.c) NH₃ (ammonia) sensor monitoring from midpoint or downstream ofcatalyst. The NH₃ sensor 76 may be used to detect when over-dosing ofreagent occurs.d) Monitoring of system temperatures including reagent, exhaust,catalyst, ambient. This may be achieved by means of the reagenttemperature sensor 25, SCR catalyst temperature sensor 74, and othersuitably placed temperature sensors.e) Monitoring of engine intake humidity.f) Monitoring of engine inlet and/or exhaust O₂ (oxygen) content toestimate Exhaust Gas Recirculation (EGR) level.g) Monitoring of reagent level in tank module from reagent level sensor24 output.h) Monitoring of reagent solution quality using reagent quality sensor26.i) Monitoring of pump stoke completion via glitch detection.j) Estimation of reagent level in tank module from cumulative valid pumpstroke count. Since a fixed volume of reagent is dispensed with eachvalid pump stroke, if the initial quantity of regent in the reagent tank21 is known, then the remaining quantity can be deduced from the totalnumber of valid pump strokes performed.k) Control of pump actuator drive energy level to achieve a validstroke.l) Control of reagent delivery quantity in unit time through injectoractuation frequency.m) Control of reagent heaters for tank module and heated lines toinjector. For example, the reagent heater 22 and/or the supply lineheater 37 may be switched on in the event that a temperature detected bythe reagent temperature sensor 25 is below a threshold value.n) Perform sensor linearizations.o) Perform system diagnostic surveys.p) Perform On-Board Diagnostic (OBD) reporting functions. Faults oranomalies in the dosing system may be reported to the user.q) Maintain system fault database.r) Communicate with vehicle/engine control module, for example, by meansof the CAN link 77 or other suitable connection.s) Communicate with off-board reagent dispensing device, for example, bymeans of the antenna 78.t) Proportional control of reagent delivery between SNCR and SCR. Incases where both selective non-catalytic reduction and selectivecatalytic reduction are employed, the DCU 70 may be operable to adjustreagent delivery accordingly.

The SCR dosing system of the present invention has all of the attributesof the system described previously.

Referring to FIG. 4, there is shown an SCR dosing system 1 of thepresent invention in which like numbers to those in FIG. 1 are used todenote similar parts. Importantly, the SCR dosing system in FIG. 4further comprises a priming pump 80 in addition to thepositive-displacement metering pump 41 of the injector module 40. Thepriming pump 80 is disposed adjacent to the reagent tank 21, and isoperable to urge reagent along the supply line 35 from the reagent tank21 toward the injector module 40.

The arrangement shown in FIG. 4 is particularly beneficial in caseswhere the reagent tank 21 is spaced far apart from the injector module40, or where the reagent tank 21 is not disposed sufficiently highenough above the injector module 40 for the reagent to flow along thesupply line 35 under the influence of gravity alone. In such cases, thesuction effect provided by the metering pump 41 may not be sufficient todraw reagent from the reagent tank 21 quickly enough to ensure that SCRof NOx in the exhaust gas stream starts soon enough after engine startto comply with legislative constraints.

The priming pump 80 need only be a low pressure pump (i.e. substantiallyless than 6 bar, and more probably only 0.5 bar). This is because, withthe SCR dosing system 1 of the present invention, it is not necessaryfor reagent in the supply line 35 to be pressurised as highly as thepressure at the injector module nozzle valve 64. Accordingly, the costof the priming pump 80 may be kept low.

The priming pump 80 may be operable at or immediately followingengine-start, i.e. during a start-up mode of the engine, in order toprime the supply line 35 with reagent. Alternatively, or in addition,the priming pump 80 may operate continuously during running of theengine, or it may be operated intermittently during running of theengine as required to ensure an adequate quantity of reagent is suppliedto the injector module 40 to ensure that a required dosing schedule ismaintained. A pressure sensor may also be provided in the supply line 35and the priming pump 80 may be operable in response to a signal from thesensor that is indicative of the pressure of reagent in the supply line35.

As described above with reference to FIGS. 1 and 2, the DCU 70 maycomprise processing means and memory means which are distinct from theECU 5 of the vehicle in which the SCR dosing system 1 is installed. Inthis case, the CAN link 77 between the DCU 70 and the ECU 5 enables datato be transferred therebetween. Accordingly, with this configuration,the DCU 70 can receive input signals from existing sensors which servethe ECU 5.

Referring to FIG. 5, in an alternative embodiment of the presentinvention, the DCU 70 may be a stand-alone unit, which is not linked tothe vehicle ECU 5. In this case, the DCU 70 may be provided with its ownarray of engine mounted input sensors, which are distinct from thosesensors which serve the ECU 5. The inputs from the engine mounted inputsensors are indicated by reference numeral 79 in FIG. 5.

The embodiment of FIG. 5 is particularly useful for after-marketapplications in which the SCR dosing system 1 is retrofitted to avehicle. In this case, installation of the SCR dosing system 1 issimplified by the fact that it is not necessary to provide the CAN link77 to the vehicle ECU 5, thereby any compatibility problems with the ECU5 are avoided. Accordingly, with the stand-alone DCU of FIG. 5, thecapability or authority of the DCU 70 may extend to full responsibilityfor all aspects of reductant dosing including NOx estimation based onengine models and/or sensors.

Referring to FIG. 6, in another embodiment of the present invention, theDCU 70 may be physically integrated into the existing engine or vehiclecontrol unit (ECU) 5. In this case, the DCU 70 may have a reduced levelof authority, such that authority is split between the DCU 70 and theECU 5.

Referring to FIG. 7, an additional injector module 90 may be provided toperform hydrocarbon (HC) dosing for regeneration of the DPF 11. The HCinjector module 90 has the same configuration as the SCR reagentinjector module 40 described previously. Thus, both the HC injectormodule 90 and the SCR injector module 40 can be controlled by the DCU70.

The HC injector module 90 is installed in the first exhaust portion 8 ofthe exhaust system 7. The HC injector module 90 is connected to an HCsupply 92, such as fuel supplied from the vehicle fuel tank. HC dosingfrom the HC injector module 90 produces an exothermal reaction in theexhaust oxidation catalyst 9 and the increase in temperature in theexhaust stream results in regeneration of the DPF 11 whereincarbonaceous particles trapped within the DPF 11 are oxidized.

With this configuration, problems associated with conventional latepost-injection in-cylinder dosing can be avoided. More specifically,where HC dosing is performed in-cylinder, after a combustion event hastaken place, fuel may adhere to the wall of the engine cylinder whichcan have an adverse effect on engine emissions performance anddurability.

This problem does not arise with the configuration of FIG. 7 and,although this configuration requires a separate HC injector module, thesystem is simplified by the fact that a single DCU 70 may control aplurality of injector modules wherein, for example, one module doses areductant for NOx remediation and the other module doses hydrocarbon(HC) for DPF regeneration.

Moreover, a single DCU may be operable to control a plurality ofinjector modules, for example in the case where a multi-bank engine hastwo separate exhaust systems with an SCR catalyst in each, as will bedescribed in more detail below.

In the following description an injector module for injecting reductantinto the exhaust gas stream for the purpose of NOx remediation isreferred to as an “SCR doser”. Also, an injector module used forinjecting HC into the exhaust gas stream for the purpose of DPFregeneration is referred to as an “HC doser”.

Referring to FIG. 8, a multi-bank engine 100, such as a “V6”configuration, comprises first and second opposed banks of enginecylinders 102, 104. The exhaust system comprises first and secondbraches 112, 114, wherein each branch is associated with one of thefirst and second banks of engine cylinders 102, 104, respectively.

The first branch of the exhaust system 112 comprises a hydrocarbon (HC)doser 116, an oxidation catalyst 118, a particulate filter 120, an SCRdoser 122 and an SCR catalyst 124. Similarly, the second branch of theexhaust system 114 comprises a hydrocarbon (HC) doser 126, an oxidationcatalyst 128, a particulate filter 130, an SCR doser 132 and an SCRcatalyst 134. The second branch 114 also includes an optional additionalSCR doser 131. An additional SCR doser may optionally be included in thefirst branch 112 as well.

It is an advantage of the above-described exhaust system layout, that asingle DCU may be employed to control SCR dosing in both the first andsecond branches 112, 114, by means of the respective SCR dosers 122,132, 131. Furthermore, each of the HC dosers 116, 126 may have the sameconstruction as the SCR dosers 122, 131, 132. In this case, the DCU 70may be operable to control the HC dosers 116, 126 as well, therebyproviding a simple and low cost system.

Referring to FIG. 9, an engine 200 having a “straight six” configurationhas an exhaust system which comprises a common exhaust portion 202, afirst branch 204 and a second branch 206. The common exhaust portion 202includes a particulate filter 208 and an SCR doser 210 disposeddownstream from the particulate filter 208. Downstream from the SCRdoser 210, the common exhaust portion 202 divides into the first andsecond branches 204, 206. The first and second branches 204, 206comprise first and second SCR catalysts 212, 214, respectively.

With the above-described arrangement, a single SCR doser 210 may beemployed to provide reagent for two separate SCR catalysts 212, 214.

Referring to FIG. 10, the exhaust system is the same as that shown inFIG. 6, with the exception that instead of the single SCR doser 210,first and second SCR dosers 215, 216 are provided in the first andsecond branches 204, 206, respectively. The first SCR doser 215 isdisposed upstream of the first SCR catalyst and, likewise, the secondSCR doser 216 is disposed upstream of the second SCR catalyst 214.

Referring to FIG. 11, the exhaust system is the same as that shown inFIG. 10, with the exception that instead of the single particulatefilter 208 in the common exhaust portion 202, first and secondparticulate filters 217, 218 are provided in the first and secondbranches 204, 206. The first particulate filter 217 is disposed upstreamof the first SCR doser 215 and, likewise, the second particulate filter218 is disposed upstream of the second SCR doser 216.

Referring to FIG. 12, the exhaust system is similar to that shown inFIG. 10, with the exception that the second branch 206 comprises an HCdoser 219 and a NOx adsorber or Lean NOx Trap (LNT) 220, rather than thesecond SCR doser 216 and second SCR catalyst 214. The HC doser 219 isdisposed upstream of the LNT 220, in order to inject HC into the exhaustgas stream flowing in the second branch 206 so as to regenerate the LNT220.

As explained previously, since the HC doser 219 can be an injectormodule having the same construction as the SCR doser 215, a single DCUcan be configured to control reagent dosing of the SCR catalyst 212 andregeneration of the LNT 220.

As shown in more detail in FIG. 13, the exhaust system also comprises aflap valve 240 disposed at the junction between the common exhaustportion 202, the first branch 204 and the second branch 206, i.e. in theregion labelled ‘A’ in FIG. 12. The flap valve 240 comprises a flap 242,which is movable about a pivot 244. The flap 242 is sized so that it canblock the mouth of either the first branch 204 or the second branch 206depending on the rotation of the flap 242 about the pivot 244.

Movement of the flap 242 is controlled by an actuator (not shown), andthe position of the flap determines whether the exhaust gas stream flowsalong the first branch 204, the second branch 206 or both the first andsecond branches 204, 206 concurrently.

Accordingly, when the flap 242 is positioned so as to block the mouth ofthe first branch 204 (as shown in FIG. 13), the exhaust gas stream isdirected along the second branch 206, such that NOx may be removed fromthe exhaust gas stream by the LNT 220. Conversely, when the flap 242 ispositioned so as to block the mouth of the second branch 206, theexhaust gas stream is directed along the first branch 204, such that NOxmay be removed from the exhaust gas stream by the SCR catalyst 212.

With the above-described configuration, it is possible to select themost appropriate method for removing NOx from the exhaust gas stream.For example, in the event that the reagent tank 21 is empty, the exhaustgas stream can be directed along the second branch 206 so that NOx isremoved using the LNT 220. Alternatively, during a regeneration event ofthe LNT 220, the exhaust gas stream may be directed along the firstbranch 204 so that NOx is removed using the SCR catalyst 212.

The flap valve 240 may be a bipolar valve, such that the flap 242 alwaysblocks one of the first and second branches 204, 206. Alternatively, theflap valve 240 may be operable such that the flap 242 can be positionedmidway between these positions such that the exhaust gas stream isdirected along both the first and second branches 204, 206. In thiscase, since both the SCR catalyst 212 and the LNT 220 are operable toremove NOx from the exhaust gas stream, each of these components may beconstructed with a smaller size than would otherwise be required. Inanother embodiment, the position of the flap valve 240 may be modulatedto adjust the proportion of exhaust gas that passes down the first orsecond passage.

Referring to FIG. 14, the exhaust system is similar to that of FIG. 11,with the exception that that the second branch 206 comprises an HC doser221 and a NOx adsorber or Lean NOx Trap (LNT) 222, rather than thesecond SCR doser 216 and second SCR catalyst 214. The HC doser 221 isdisposed upstream of the LNT 222, in order to inject HC into the exhaustgas stream flowing in the second branch 206 so as to regenerate the LNT222. Accordingly, the exhaust system may be provided with a flap valve240 of the kind shown in FIG. 13, and may operate in the same way as theembodiment shown in FIG. 12.

Referring to FIG. 15, an exhaust system comprises a single branch 224having an HC doser 225, an LNT 226, an SCR doser 227 and an SCR catalyst228. The HC doser 225 is disposed upstream of the LNT 226, in order toinject HC into the exhaust gas stream so as to regenerate the LNT 226.The SCR doser 227 is disposed downstream of the LNT 226, and the SCRcatalyst 228 is disposed downstream of the SCR doser 227. With thisconfiguration remediation of NOx in the exhaust gas stream may beimproved at lower operating temperatures, e.g. following engine start.More specifically, the LNT 226 is operable to trap NOx in the exhaustgas stream before the SCR 228 catalyst reaches its normal operatingtemperature of about 200° C. Accordingly, at a temperature of, forexample, 150° C., NOx is removed from the exhaust gas stream by the LNT226, resulting in lower NOx emissions from the exhaust system than couldbe achieved using the SCR catalyst 228 alone.

Another advantage of the above-described configuration is that the LNT226 can be used to produce NH₃. Accordingly, the NH₃ produced by the LNT226 can supplement the reagent injected by the SCR doser 227 in order toremediate NOx in the SCR catalyst 228. Thus, it is possible to reducethe amount of reagent which is required to be injected by the SCR doser227. Further details of ammonia generation techniques can be found inour co-pending US patent applications US-2007-0065354 andUS-2007-0271908.

Referring to FIG. 16, the exhaust system comprises a common exhaustportion, 230, which divides into a first branch 231 and a second branch232. A 3-way valve 240 of the type described with reference to FIG. 13is disposed at the intersection between the common exhaust portion, thefirst branch and the second branch.

The first branch 231 comprises an HC doser 233 and an LNT 234 disposeddownstream of the HC doser 233. The second branch 232 comprises an SCRdoser 235 and an SCR catalyst 236 disposed downstream of the SCR doser235. A downstream end of the first branch 231 intersects with the secondbranch 232 at a point upstream of the SCR doser 235 and downstream ofthe valve 240, such that the exhaust gas stream in the first branch 231can flow into the second branch 232.

The valve 240 is operable to direct the exhaust gas stream flowing inthe common exhaust portion 230 into an upstream end of either the firstbranch 231 or the second branch 232 or, optionally, into the first andsecond branches 231, 232.

In the case that the valve 240 directs the exhaust gas stream into thefirst branch 231 only, the exhaust gas flows through the LNT 234 intothe second branch 232 and through the SCR catalyst 236. Accordingly, asdescribed previously with reference to the embodiment shown in FIG. 15,this arrangement has the advantage that NOx may be removed by the LNT234 before the SCR catalyst 236 has reached its normal operatingtemperature. Additionally, NH₃ produced by the LNT 234 can be used tosupplement the reagent injected by the SCR doser 235.

In the case that the valve 240 directs the exhaust gas stream into thesecond branch 232 only, the exhaust gas flows only through the SCRcatalyst 236. With this configuration, when the SCR catalyst has warmedup to its normal operating temperature the LNT 234 may not be requiredand the exhaust gas can be directed through the SCR catalyst 236 only.Alternatively, during regeneration of the LNT 234 when the HC doser isinjecting, it may be required that the exhaust gas stream is directedalong the second branch 232 only.

Referring to FIG. 17, in order to comply with legislation, NOx emissionsmust be controlled over a specified driving/emissions test cycle oremissions zone 300. The emissions zone 300 covers a range of engineloads and engine speeds. Accordingly, the SCR dosing system 1 must beoperational across the emissions zone 300. When operating at high loadconditions, beyond the zone covered by the test cycle, reagent dosing isnot strictly required and in the interests of minimizing reagentconsumption, the vehicle calibrator may elect to stop dosing even thoughNOx is being generated by the engine and released to the atmosphere. Theabove-described SCR injector module 40 employs a thermal managementmeans including an insulated sleeve for the dispenser 42 where it entersthe exhaust system 7. This beneficial insulation arrangement issufficient to ensure that additional dosing across the emissions zonefor cooling purposes is not required. Nevertheless, when operating athigh load, exhaust temperatures may be in excess of 500° C. and extendedtime at this condition will overwhelm the insulating measures for thedispenser 42 causing the aqueous urea to dry out and clog the dispenser42 with precipitated salts.

In an embodiment of the present invention, the DCU 70 controls theinjector module 40 to perform intermittent or sporadic injections ofreagent under high load conditions, even though the calibration does notspecifically require such dosing. Accordingly, the insulated connectingpipe 43 between the metering pump 41 and the dispenser 42 may be keptclear and cool by the intermittent flow of reagent. A specific featureof the solenoid-operated positive displacement pump is that theinjection pressure is significantly higher than that from known dosingsystems. Thus, in the case of the present invention, the metering pump41 is able to force fresh reagent through the precipitated salts suchthat dosing performance may be maintained under conditions that woulddefeat prior art systems.

In one aspect of the present invention, when operating in a thermalmanagement zone 310, which is a speed/load zone that is both outside andof higher load than that covered by the emissions cycle 300, and wherethe normal calibration may call for no or a severely restricted reagentdosing schedule, a flow management regime is employed in which thedosing schedule is based upon either measured or estimated exhaust gastemperature in the vicinity of the reagent atomizer rather than upon thenormal NOx reduction strategy. Thus, the higher the exhaust gastemperature adjacent the dispenser 42, the higher the metering pumpactuation repetition rate, with the rate being determined duringdevelopment by the ability of the injection module 40 to maintaincorrect function as soon as the load returns into the emissions cycleregime. During this flow management regime, the dosing rate issignificantly lower than that during the emissions cycle regime sincethe target is to prevent dry-out and clogging of the atomizer and notnecessarily to keep NOx emissions within the legislated levels.

Typically, during the emissions cycle 300, the dosing frequency isbetween 0 and 100 Hz, and usually around 100 Hz, and is determined bythe SCR catalyst temperature and the NH₃ loading. The optimum dosingrate is determined by the engine NOx production, which in turn relatesto engine load. In thermal management zone 310, the dosing frequency istypically between 0 and 20 Hz and is determined substantially by theexhaust temperature. At these relatively high frequencies (typically20-100 Hz), dosing is therefore fairly continuous but with a variablefrequency depending on engine load (lower frequency at light load,higher frequency as high load) and whether the engine is running in theemissions cycle 30 or in the thermal management zone 310. It will beappreciated that even while dosing in the thermal management zone 310,there will also be some impact on NOx reduction across the SCR catalyst,but to a much lower extent than during the normal emissions cycle 300.

Furthermore, the above-described flow management regime may beconveniently combined with a glitch detection strategy, wherein glitchdetection is used to distinguish valid from non-valid pumping strokes.More specifically, referring to FIG. 18, rather than use exhausttemperature as an input as described above, the “time slope” of thepumping event may be monitored. Accordingly, as the dispenser 42 beginsto clog, the time interval from energization to glitch detection willincrease as it becomes harder to displace a shot of reagent. As apredetermined time limit is approached, the control algorithm increasesthe repetition rate to maintain a free-flowing duct.

Legislative bodies, such as the United States Environmental ProtectionAgency, may prescribe a maximum number of engine restarts which arepermitted after the reagent tank runs dry, e.g. <20 engine restarts.Under these circumstances, it would not be possible to implement theabove-described strategy with a dry tank. Accordingly, regular dosingmay be stopped just prior to exhaustion of the tank so that theremaining contents of the tank can be used for the above thermalmanagement strategy as necessary.

Additionally, the thermal management strategy may be linked with DPFregeneration events that also generate very high exhaust temperaturesupstream of the injection module. For example, when the DCU determinesthat a DPF regeneration is taking place, the reagent can be under-dosedon the basis that it will be necessary to over-dose later in the eventwhen temperatures are higher, whilst always trying to avoid ammonia slipfrom the SCR catalyst.

An objective of the total SCR system of which the reagent dosing system1 is a part, is to maximize NOx conversion into harmless gases asefficiently as possible, particularly over the transient emissionsdriving cycle. Due to catalyst limitations, this conversion can onlytake place over the substrate temperature range of circa 180 to 450° C.,but these conditions cover most of the engine operating range in termsof speed and load. Nevertheless, in covering this range, there is a widevariation in exhaust gas space-velocity at the reagent entry point intothe exhaust system 7. The purpose of the dispenser 42 is to mix thereagent with the exhaust gas stream as homogeneously as possible so thatit hydrolyses as rapidly as possible and the resulting NH₃ is depositedacross the face of the SCR catalyst 13 as uniformly as possible. If thisobjective is achieved, then the necessary mixing length can be shorterthan would otherwise be the case and, due to the more efficientutilization, the SCR catalyst 13 may be smaller also. Both outcomes arehighly desired.

However, a dispenser whose spray characteristics (in terms of dropletsize and momentum) have, for example, been optimized for a moderateexhaust temperature and space-velocity will under-penetrate and be tooeasily deflected to achieve uniform mixing at other engine conditionsthat may provide a high space velocity, and vice versa. Therefore it isdesirable to obtain a good match between the spray characteristics andthe exhaust flow conditions across the operating range of the engine.

In an embodiment of the present invention, the above objective isachieved through the use of a “smart” actuator drive circuit. Asexplained previously, drive current glitch detection may be used toestablish the time interval between the start of energization and thecompletion of a full and valid pumping stroke of the plunger 58.Assuming normal hydraulic conditions, i.e. free-flowing reagent with thesupply line 35 not frozen or clogged, then the time to achieve a validstroke will, to a large extent, depend on the energy that is expended bythe actuator during the pumping event. Likewise there will be arelationship between the energy delivered by the actuator to themetering pump 41 and the reagent injection pressure generated at thenozzle valve 64, and in turn between the injection pressure and thedroplet size and the momentum imparted to the spray. Therefore, byincreasing or decreasing the drive current waveform it becomes possibleto influence the spray momentum and thus provide flexibility in matchingreagent spray characteristics to the prevailing exhaust gasspace-velocity.

Thus, in use, it is proposed to vary the drive current level as afunction of exhaust gas space velocity, the latter value being derivedfrom an engine speed/load map or from an engine model, with higher drivecurrent being provided at times of higher space velocity and vice versa.Variation of the drive current may be achieved using a voltage-choppeddrive whereby the voltage chop frequency controls the resultant currentlevel; a lower chop frequency results in higher drive current, and viceversa. However, changing the actuator drive current level also changesthe time interval between actuator energization and detection of thevalid stroke current glitch; a lower drive current will result in alonger elapsed time for completion of a valid stroke, and vice versa.This means that the “valid stroke window” will also need to change withthe current level. Further details of this glitch detection techniquecan be found in our co-pending U.S. patent application Ser. No.10/879,210.

One of the potential problems of the solenoid operated pump concept isthe issue of noise generated by the armature 61 striking the lift-stopat either end of its stroke. Again, the smart drive box may be used toameliorate this problem. Employing the glitch-detect feature, it becomespossible to use an adaptive algorithm to estimate the timing of theglitch event and then turn off the drive current just prior toend-of-stroke so that a full and valid stroke is obtained but with asoft landing of the armature 61. Likewise, on the spring-biased returnstroke, a soft landing can be arranged with a short duration drive pulsethat is timed to arrest its landing velocity.

The present invention is particularly suitable for use with light dutyvehicles with compression ignition (diesel) engines. However, it will beappreciated by those skilled in the art that the present invention mayequally be applied to other lean-burn engines operating on variousfuels, such as gasoline, since they too may utilize SCR technology.

It will be appreciated by those skilled in the art that there is morethan one NOx-reducing catalyst/reagent combination. While urea SCR iscurrently favoured for automotive use due to its favourable conversionratio versus catalyst temperature performance, other combinations suchas HC (hydrocarbon) SCR may equally be performed by an SCR dosing systemaccording to the present invention.

1. A method of dosing a reagent into an exhaust gas stream of aninternal combustion engine having an SCR catalyst, the methodcomprising: injecting reagent from a reagent tank into the exhaust gasstream at a position upstream of the SCR catalyst using a reagentinjector (40) in accordance with a first dosing schedule in order toremediate a predetermined proportion of NOx in the exhaust gas stream,the first dosing schedule being associated with a first range of engineoperating conditions; injecting reagent from the reagent tank into theexhaust gas stream at a position upstream of the SCR catalyst using areagent injector in accordance with a second dosing schedule in order toenable heat transfer between the reagent injector and said injectedreagent, the second dosing schedule being associated with a second rangeof engine operating conditions; and dosing in accordance with said firstor said second dosing schedule in dependence on whether engine operatingconditions lie within said first or said second range of engineoperating conditions; wherein the proportion of NOx in the exhaust gasstream which is remediated by dosing using said second dosing scheduleis less than said predetermined portion.
 2. A method according to claim1, wherein said second range of engine operating conditions ischaracteristic of higher engine load and/or speed than said first rangeof engine operating conditions.
 3. A method according to claim 1,further comprising: sensing the level of reagent in the reagent tank;and halting the first dosing schedule when the sensed level of reagentdrops below a predetermined level.
 4. A method according claim 1,wherein the injector comprises an atomising dispenser and apositive-displacement metering pump, the method further comprisingdrawing reagent from the reagent tank by means of the metering pump anddelivering it to the dispenser.
 5. A reagent dosing system for dosing areagent into the exhaust gas stream of an internal combustion engine,the system comprising: a reagent tank for storing a supply of reagent;an injector module comprising an atomising dispenser and apositive-displacement metering pump which draws reagent from the reagenttank and delivers it to the dispenser; a supply line coupling thereagent tank to the injector module; a dosing control unit operable tocontrol the injector module to inject reagent into the exhaust gasstream; and an additional priming pump arranged, in use, to urge reagentalong the supply line toward the injector module under selectedconditions.
 6. A system according to claim 5, wherein the dispenser andthe metering pump are integrated within the same unit.
 7. A systemaccording to claim 5, wherein the priming pump is operable at or duringa start-up mode.
 8. A system according to claim 5, wherein the primingpump is operable continuously or intermittently during running of theengine.
 9. A system according to claim 5, further comprising a tankmodule mountable within the reagent tank, said tank module comprising: areservoir comprising a tubular member with a first open end and a secondclosed end, the second end having an opening therein to enable theinflow of reagent from the reagent tank; a closure member for closingthe first end of the tubular member to prevent the flow of reagenttherefrom; and a reagent pickup tube having a first end disposed withinsaid reservoir and a second end in fluid communication with said supplyline; wherein the priming pump is disposed within the tank module.
 10. Amethod of dosing a reagent into the exhaust gas stream of an internalcombustion engine having an SCR catalyst, the method comprising:injecting a spray of reagent with a spray momentum into the exhaust gasstream using an injector module comprising a positive-displacementmetering pump and an atomising dispenser; controlling thepositive-displacement metering pump in accordance with a controlparameter which is dependent on the exhaust gas stream space velocity sothat the spray momentum provides optimum mixing of the injected reagentwith the exhaust gas stream upstream of the SCR catalyst.
 11. A methodas claimed in claim 10, wherein the control parameter is a drive currentlevel.
 12. An exhaust system for an internal combustion enginecomprising: a common exhaust portion having a first end coupled to theengine for receiving an exhaust gas stream emitted therefrom and asecond end which divides into first and second branches, the commonexhaust portion comprising a particulate filter and said first andsecond branches comprising first and second SCR catalysts respectively;and an SCR doser for injecting reagent into the exhaust gas stream at alocation disposed between said particulate filter and said second end ofthe common exhaust portion.
 13. An exhaust system for an internalcombustion engine comprising: a common exhaust portion having a firstend coupled to the engine for receiving an exhaust gas stream emittedtherefrom and a second end which divides into first and second branches,the common exhaust portion comprising a particulate filter and saidfirst and second branches comprising first and second SCR catalysts,respectively; a first SCR doser for injecting reagent into said firstbranch at a location disposed upstream of said first SCR catalyst; and asecond SCR doser for injecting reagent into said second branch at alocation disposed upstream of said second SCR catalyst.
 14. An exhaustsystem for an internal combustion engine comprising: a common exhaustportion having a first end coupled to the engine for receiving anexhaust gas stream emitted therefrom and a second end which divides intofirst and second branches; said first branch comprising a firstparticulate filter, a first SCR catalyst and a first SCR doser forinjecting reagent into said first branch at a location disposed betweensaid first particulate filter and said first SCR catalyst; and saidsecond branch comprising a second particulate filter, a second SCRcatalyst and a second SCR doser for injecting reagent into said secondbranch at a location disposed between said second particulate filter andsaid second SCR catalyst.
 15. An exhaust system for an internalcombustion engine comprising: a common exhaust portion comprising aparticulate filter and having a first end coupled to the engine forreceiving an exhaust gas stream emitted therefrom and a second end whichdivides into first and second branches; wherein the first branchcomprises an SCR catalyst and an SCR doser for injecting reagent intosaid first branch at a location disposed upstream of said SCR catalyst;and the second branch comprises a Lean NOx Trap and a hydrocarbon doserfor injecting hydrocarbons into said second branch at a locationdisposed upstream of said Lean NOx Trap.
 16. An exhaust system for aninternal combustion engine comprising: a common exhaust portion having afirst end coupled to the engine for receiving an exhaust gas streamemitted therefrom and a second end which divides into first and secondbranches; wherein the first branch comprises a first particulate filter,an SCR catalyst and an SCR doser for injecting reagent into said firstbranch at a location disposed between said first particulate filter andsaid SCR catalyst; and the second branch comprises a second particulatefilter, a Lean NOx Trap and a hydrocarbon doser for injectinghydrocarbons into said second branch at a location disposed between saidsecond particulate filter and said Lean NOx Trap.
 17. An exhaust systemaccording to claim 15, comprising a 3-way valve disposed at said secondend of said common exhaust portion, said valve being operable to directsaid exhaust gas stream along said first and/or said second branch. 18.An exhaust system for an internal combustion engine comprising: a commonexhaust portion having a first end coupled to the engine for receivingan exhaust gas stream emitted therefrom and a second end which dividesinto first and second branches; a 3-way valve disposed at said secondend of said common exhaust portion, said valve being operable to directsaid exhaust gas stream along said first and/or said second branch;wherein the first branch comprises a hydrocarbon doser for injectinghydrocarbons into said first branch and a Lean NOx Trap disposeddownstream of said HC doser; the second branch comprises an SCR doserfor injecting reagent into said second branch and an SCR catalystdisposed downstream of said SCR doser; and a downstream end of saidfirst branch is coupled to said second branch at a location disposedupstream of said SCR doser, to enable the flow of exhaust gasestherebetween.